1. Field of the Invention
The invention relates to information storage media, in particular media useful with holographic storage systems.
2. Discussion of the Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Page-wise systems involve the storage and readout of entire two-dimensional pages of data. Specifically, recording light passes through a two-dimensional array of dark and transparent areas representing a page of data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index and/or absorption imprinted into a storage medium. Holographic systems are discussed generally in D. Psaltis et al., "Holographic Memories," Scientific American, November 1995, the disclosure of which is hereby incorporated by reference. One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which is hereby incorporated by reference. In phase correlation multiplex holography, a reference light beam is passed through a phase mask, and intersected in the recording medium with a signal beam that has passed through an array representing data, thereby forming a hologram in the medium. The relation of the phase mask and the reference beam is adjusted for each successive page of data, thereby modulating the phase of the reference beam and allowing the data to be stored at overlapping areas in the medium. The data is later reconstructed by passing a reference beam through the original storage location with the same phase modulation used during data storage.
The capabilities of holographic storage systems are limited in part by the storage media. Iron-doped lithium niobate has been used as a storage media for research purposes for many years. However, lithium niobate is expensive, exhibits poor sensitivity, and tends to introduce noise during read-out of the stored information. Alternatives have therefore been sought, particularly in the area of photosensitive polymer films. See, e.g., W. K. Smothers et al., "Photopolymers for Holography," SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif. 1990. The material described in this article contains a photoimageable system of a liquid monomer material and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), in a matrix organic polymer that is substantially inert to the exposure light. During writing of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the resultant lowering of the monomer concentration, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient create a refractive index change, forming the hologram containing the data. Unfortunately, deposition of the pre-formed matrix material containing the photoimageable system requires use of solvent, and the thickness of the material is therefore limited, e.g., to no more than about 150 .mu.m, to allow adequate evaporation of the solvent. In addition, polymerization-induced bulk shrinkage of 4-10% detrimentally affects the reliability of data retrieval (bulk indicating that the shrinkage occurs through the whole of the material).
U.S. patent application Ser. No. 08/698,142 (our reference Colvin-Harris-Katz-Schilling 1-2-16-10), the disclosure of which is hereby incorporated by reference, relates to a polymeric holographic medium. The medium is formed by mixing oligomeric matrix precursor and photoactive monomer, and photo-curing the mixture such that (a) the matrix is formed from the oligomer, and (b) at least a portion of the monomer remains unreacted and thus available for holographic recording. Because no solvent is required for deposition of these materials (since the mixture is fluid), greater thicknesses are possible, e.g., 1 mm and above. Although the material has many attributes, greater resolution would be possible if bulk shrinkage of the medium during writing could be limited.
Attempts to provide a photoimageable system containing a monomer in a glass matrix have also been made. U.S. Pat. No. 4,842,968 discloses a porous glass matrix in which the pores are filled with a photoimageable system. The porous glass is placed into a container of the photoimageable system, where the photoimageable system diffuses into the pores. After exposure to light, the unexposed, i.e., non-polymerized, portions of the photoimageable system must be removed from the pores with a solvent, and a different material is typically substituted into the emptied pores. The glass matrix offers desirable structural integrity, in contrast to media containing polymer matrices, and also allows formation of relatively thick, e.g., greater than 1 mm, photorecording media useful for holographic storage systems. However, the medium of this patent has several practical drawbacks. Specifically, complex chemical treatments are required to remove unreacted monomer, and there is small latitude in defining the structure, volume percent porosity, pore size, and pore interconnectivity in the preformed glass.
Thus, while progress has been made in fabricating photorecording media suitable for use in holographic storage systems, further progress is needed. In particular, media which have improved chemical and structural integrity, which are capable of being formed in relatively thick, e.g., greater than 1 mm, layers without complex chemical treatments, and which allow some latitude in selecting matrix properties, are desired.